Experimental study on soil-water characteristic curves of subgrade coal gangue filler

As a waste material generated during coal mining and washing, coal gangue is often used for filling roadbeds. The compacted coal gangue has a dual-porosity structure, which usually leads to bimodal soil-water characteristic curves (SWCC). SWCC is closely related to the permeability coefficient of unsaturated soils, which has an important influence on the water migration inside the roadbed and the long-term serviceability of the roadbed. In order to investigate the influence of compaction degree, silt content and maximum particle size on bimodal SWCCs, a series of pressure plate tests was conducted on compacted coal gangue samples. The experimental data was fitted by a bimodal SWCC function model, and the variation of fitted parameters was analyzed. The test results show that when the silt content is 5%, samples with different compaction degrees have bimodal SWCCs. When the silt content is 50%, the SWCCs of samples change from bimodal to unimodal with the increase of compaction degree, so it can be inferred that there exists a critical value of compactness between 0.90 and 0.95. Increasing compaction degree or silt content, or decreasing maximum particle size will reduce the difference of step height in SWCCs. With the increase of compaction degree or silt content, the values of model parameters alpha(1), alpha(2) (parameters related to the air-entry value) and n(1), n(2) (parameters related to drainage degree) decrease gradually, that is, the air-entry values and the water holding capacity of both macro-pore part and micro-pore part increase.